1. Field of the Invention
The present invention relates to radiant-temperature measurement technologies, in particular to radiant-temperature measurement devices, light-source temperature control devices, and image projection apparatus, as well as to radiant-temperature measurement methods.
2. Description of the Related Art
A spectrum according to thermal radiation or light emitted from a body depends on the temperature thereof. There is one of the methods that utilize spectrum change so as to measure the temperature, i.e. a two-color (ratio) or dual-wavelength radiant-temperature measurement method, in which temperature is obtained based on an intensity ratio of thermal radiations in two different wavelength bands; the method enables obtaining the temperature regardless of emissivity of the thermal radiation from a body.
However, in addition to the thermal radiation (light) from the body, when other light is superimposed as stray light, an error is imposed on the value of a temperature obtained. For example, in a case for which a discharge lamp is used as a light source, and when the temperature of electrode portions that are light-source's light-emitting ends is measured, because of strong discharge-light emission existing near by, the thermal radiation (light) from the electrode portions is resultantly measured with the discharge-light emission being superimposed thereon as the stray light. Therefore, in order to carry out the temperature measurement based on a thermal radiation spectrum, it is necessary to remove an influential component (or the amount of influence) caused by the stray light out from the measured value. As one means for removing an influential component owing to the stray light, there exists an invention described in Japanese Patent Application Publication No. JP1996-152360 (1996), Page 5, FIG. 1. In the invention, in order to remove the influential component owing to discharge-light emission, two wavelength bands are selected so as to exclude spectral peaks, of the discharge-light emission, causing the stray light, so that a dual-wavelength radiant-temperature measurement method is applied.
By measuring thermal radiation in a wavelength band excluding spectral peaks of the discharge-light emission, it is possible to eliminate an influence caused by the stray light up to a certain extent. However, actual spectra of discharge-light emission accompany continuous spectra over a wide range of wavelengths other than those in line spectra. As an example, spectra of discharge-light emission when an extra-high pressure mercury lamp is used as a discharge lamp are shown in FIG. 15. It should be understood that there exits discharge-light emission of near 700 nm wavelength when those peaks of line spectra are excluded.
As a purpose of measuring the electrode temperature of a lamp, a procedure is raised in which the characteristics of the lamp are investigated so as to extend its operating life. For this purpose, it is necessary to measure temperature of an electrode's tip or end, namely, the temperature of an electrode's portion in the very close proximity of which strong discharge-light emission exists. At such a portion, even when a tow-color (ratio) or dual-wavelength temperature measurement is carried out excluding wavelength bands in which discharge-light peaks exist, a stray-light component caused by discharge-light emission has been too large to achieve an accurate measurement of the portion's temperature. As described above, spectra of the stray light intended to be removed more or less have continuous components. For this reason, an influence caused by the stray light cannot be completely removed; because remaining influential components cannot be ignored, there has been a problem in that an accurate temperature measurement cannot be made.
The present invention has been directed at solving these problems described above, and an object of the invention is to provide a temperature measurement device and its method by removing stray-light components from a measured value of thermal radiation (light) on which stray light is superimposed.
In general, because there has not been a means for measuring electrode's temperature of a discharge lamp in real time, the lamp cannot be driven while the electrode's temperature is being controlled.
In addition, although a discharge lamp is generally used for an image projection apparatus as a light source, because the electrode's temperature cannot be controlled at a constant level, there has been a problem in that useful life of the discharge lamp is shortened, requiring frequent replacement of the lamp.